1. Field of the Invention
The present invention relates to a dielectric resonator, a dielectric filter, and a duplexer for use in the microwave or millimeter wave range and also to a communication device using such an element.
2. Description of the Related Art
In recent years, with the increasing popularity of mobile communications systems and multimedia, there are increasing needs for high-speed and high-capacity communications systems. As the quantity of information transmitted via these communications systems increases, the frequency range used in communications is being expanded and increased from the microwave range to the millimeter wave range. Although TE01.delta.-mode dielectric resonators, which are widely used in the microwave range, can also be used in the millimeter waver range, extremely high accuracy is required in forming resonators because the dimensions of the cylindrical dielectric of the resonator, which determine the resonant frequency of the resonator, become very small in the millimeter wave range. In the case where a filter for use in the millimeter wave range is constructed using TE01.delta.-mode dielectric resonators, extremely high positioning accuracy is required when TE01.delta.-mode dielectric resonators are disposed at properly spaced locations in a waveguide. Furthermore, the resonance frequency of each resonator should be adjusted precisely. It is also required that coupling among dielectric resonators be precisely adjusted. However, a very complicated structure is required to perform precise adjustment.
The applicant for the present invention has proposed, in Japanese Patent Application No. 7-62625, a dielectric resonator and a bandpass filter which does not have the above problems.
FIGS. 10A and 10B illustrate the structure of the dielectric resonator disclosed in the patent application cited above, wherein only the essential parts are shown in the figure. In FIGS. 10A and 10B, reference numeral 3 denotes a dielectric substrate having a particular relative dielectric constant. Electrodes 1 and 2 are formed on both principal surfaces of the dielectric substrate 3 such that each electrode has a circular-shaped non-electrode part 4 or 5 whose diameter is properly determined. Conductive plates 17 and 18 are disposed at opposite sides of the dielectric substrate 3 so that they are spaced by a proper distance from the dielectric substrate 3. In this structure, a resonator region 60 with a cylindrical shape is formed in the dielectric substrate 3 and it acts as a TE010-mode dielectric resonator.
In the above dielectric resonator having the structure including electrodes having non-electrode parts with substantially the same shape which are formed on opposite principal surfaces of the dielectric plate disposed between the two conductive plates spaced from each other, spurious waves in a TE mode are generated between the respective electrodes on the principal surfaces of the dielectric plate and the corresponding conductive plates, and the spurious waves propagate in the spaces between the principal surfaces of the dielectric plate and the conductive plates. The spurious waves are reflected by a cavity wall and thus standing waves are generated. This means that resonance associated with such standing waves occurs.
If such TE-mode spurious waves are generated and propagate in the spaces between the respective principal surfaces of the dielectric plate and the conductive plates, energy of TE010-mode resonance which is essential in this dielectric resonator is partially transferred to energy of the spurious waves, and thus the unloaded Q (Qo) becomes low and degradation occurs in the characteristics in the frequency ranges out of the passband of the bandpass filter.
One technique for constructing a dielectric resonator and a bandpass filter which do not have the above problems has been proposed by the applicant for the present invention as disclosed in Japanese Patent Application No. 8-54452.
It is an object of the present invention to provide a dielectric resonator, a dielectric filter, a duplexer, and a communication device using such an element, in which the above-described problems are prevented in a different manner from that employed in Japanese Patent Application No. 8-54452.